DENVER GUIDE

When Denver radiators stay cold

The thermostat is calling and the cast-iron radiator in the hall is still cold. Or every radiator is cold and the boiler in the basement is clicking, locked out, or silent. Hot-water and steam boilers still heat a lot of older Denver houses, and they do not fail the way a forced-air furnace fails. These notes are the checks that stay outside the burner, the difference between water and steam, and the moment the visit should be a person who works on boilers.

By Mike Maziarz · Sources checked October 12, 2026 · How we write these guides

Radiators are not a furnace closet

A forced-air furnace pushes warm air out of registers. A boiler heats water, or makes steam, and sends it through pipes to radiators, baseboard, or a floor loop. Cold metal and quiet pipes are the symptom on this page. A blower that never starts is a different machine. If this house has supply registers and a furnace cabinet, the furnace will not ignite notes are the ones that match. If you can put a hand on a radiator that stayed cold after a real call for heat, stay here.

Older brick in Capitol Hill, Washington Park, and the Highlands still turns up with boilers. Newer infill on the same blocks often does not. Do not guess from the masonry or from what the neighbor’s house has. Look for a radiator, a fin-tube baseboard, or a boiler in the basement with pipes instead of a large sheet-metal supply trunk. Photograph the appliance from the aisle, nameplate included, without removing a cover.

A heat pump on the side yard will not warm a cast-iron radiator. Long runtimes, defrost vapor, and backup heat belong on the heat pump page. The equipment overview is the short version of the same split: furnace, boiler, or heat pump, named before anyone opens a panel. Say which one this house has when you call. A company that only talks about furnaces has not heard the complaint yet.

Smell gas and the checklist ends

A gas odor at the boiler, in the stair, or in a basement mechanical room ends every other step on this page. Get people out. Do not flip wall switches, the boiler disconnect, or the thermostat on the way. From outside, call the emergency number printed on the gas bill. Xcel Energy’s published Colorado gas tariff lists Denver among the localities where it supplies natural gas, and Xcel Energy is the name on many of those bills. This page does not print a replacement number.

Do not hunt for the leak with the boiler door open and the lights clicking. Do not decide the odor is weak enough to ignore because a radiator was cold last winter too. A failed ignition and a gas smell can arrive together. The utility call comes first. A heating company visit comes after the building is cleared. Neighbors in a shared-wall house still leave their own unit. You are not the person who verifies the pipe.

If there is no gas odor and no alarm, you can keep observing from the hallway or the top of the basement stair. The moment either one appears, the observation is over. A carbon monoxide alarm, or a headache, dizziness, or flu-like illness that eases outdoors, is the same kind of exit. Get everyone out and call emergency services. The carbon monoxide page is the longer version. Fuel-burning heat stays off until someone qualified says otherwise.

Checks that stay outside the burner

With no gas smell, read the thermostat before you condemn the boiler. Heat mode, a setpoint above the temperature the wall is actually reading, and a schedule that is not holding a deep setback are the first three facts. Batteries, if the model uses them, are a fair swap once. One breaker reset is enough if you are sure it is the boiler breaker. A second trip means the breaker stays off. The do-it-yourself or call a pro line is the same here as on a furnace: filters and thermostat batteries are house care. Gas valves and burners are not.

On a hot-water radiator, a small bleed valve at the top is sometimes a homeowner’s tool. Use it only if you already know that valve, you have a cup and a rag, and you are not opening anything on the boiler itself. Open it a little, catch the sputter of air and then water, and close it as soon as the water runs steady. If water will not stop, if you do not know which fitting you are holding, or if the valve is down at the pipe rather than the little one on the radiator, stop. Do not bleed every radiator in the house to build a project. One cold radiator and a house full of warm ones is a note. A house full of cold ones is a different note.

If a pressure gauge is visible without removing a cover, write down the number it shows while the system is cold and, if it changes, the number after a call for heat. Compare those readings only to the range printed on this boiler or in the booklet that is still on a shelf. This page will not invent a correct pounds-per-square-inch figure. A gauge buried at the bottom, or one you cannot see without a panel off, stays unread. Listen at the boiler when the thermostat calls. A small circulator pump often hums or ticks when it starts. Silence from that pump, with the burner either running or not, is worth saying out loud. Do not drain the system to see what happens. Do not add water from a hose unless the manufacturer’s instructions in your hand tell you the fill valve and you are sure the boiler is a hot-water boiler, not a steam boiler.

Hot water and steam are different machines

Hot-water boilers move water with a pump. Air trapped in a radiator, a pump that does not run, a zone valve that does not open, or pressure that has fallen off are the usual homeowner-visible versions of 'no heat.' Steam boilers make steam and do not use that same pump story. A sight glass on a steam boiler, if you can see it from the aisle, shows a water line. Write down whether you see water in the glass, whether the glass is stained so you cannot tell, or whether you do not have a glass at all. Do not add water to a steam boiler that is firing, or to one that is very hot, because you are not the person who should be charging a hot pressure vessel.

Banging pipes are a note, not a dare. A hammer in a hot-water line and a hammer in a steam main are different faults, and guessing which one you have is how a bleed key ends up on the wrong valve. If you cannot tell steam from hot water by a radiator that has a bleed screw versus a vent that hisses, say that you cannot tell. A technician can. Opening the wrong fitting on a steaming radiator can throw hot water. Leave vents, pressure controls, and the burner door alone.

One room cold and the others warm often means air in that radiator, a closed valve at that radiator, or a zone that did not open. Every room cold usually means the boiler, the pump, the water level, or a control that gave up. Those are different calls. Write which rooms lagged and whether any radiator got even lukewarm. A radiator that is hot at the bottom and cold at the top is a different sentence from a radiator that is cold all over. You do not need to name the part. You need the pattern.

Elevation, and what the city publishes

A U.S. Geological Survey table of city elevations lists Denver from 5,130 to 5,470 feet. That is the whole city sitting well above a mile, not a single porch step. Combustion air at that height is not the same density as air at a coastal bench, and a gas boiler can be set up with that fact in mind. Your question is the same one this site asks about a furnace: is this boiler set up with the elevation in mind, and how can you tell from what you see on this visit? Listen for a plain answer about this appliance. You are not collecting an orifice size or a chart value. This guide will not supply those figures.

Denver Community Planning and Development publishes mechanical-certificate scopes on denvergov.org. The boilermaker scope on that page is to install, assemble, or repair steam and hot-water boilers, along with pressure vessels and related stacks and casings. That is the city’s description, not a fee and not a promise that every dripping air vent needs a permit. Ask the office, for this address and this scope, whether the work needs a permit and which certificate covers it. Aurora, Lakewood, and Arvada keep their own counters. A Denver conversation stops at the Denver line. The permits page is how to ask without treating a slogan as an answer.

Do not let a general lecture about altitude replace a look at the flame, the vent, the water level, and the pump on the boiler in front of you. A technician who will not say what they saw is not finished, even if the radiator is warm again when they leave. Write their words. If the visit turns into a replacement pitch before anyone has named the failed step, the repair or replace notes are the brake. A single cold morning is not a new boiler.

When cold radiators mean a call

Call when the whole house is cold and a filter-style trick does not exist, because a boiler does not have the slide-out filter a furnace has. Call when the burner locks out and a single reset, if the manufacturer switch is one you already understand, does not hold. Call when water is leaking from the boiler itself, not a few drops from a radiator vent you just closed. Call when you see soot, a flame that rolls out of the draft hood, or a vent joint that has come apart. Photograph the vent from a stance that does not put your face in it. Do not tape a flue as a lasting repair.

A house that is safe, with people who can tolerate a cool night, can sometimes wait until morning. A house that is still falling, with an infant, an older adult, or anyone who gets sick in the cold, often cannot. The no-heat page sorts that difference, including what temporary heat is allowed and what is not. An oven, a grill, or a car in a closed garage is not temporary heat. A listed electric heater, plugged in directly and kept clear of fabric, is a bridge. It is not a repair.

When someone does come, ask them to name what was cold: the boiler, the pump, one zone, or one radiator. Ask what they changed and what they only bled. Ask whether elevation setup came up for this gas valve. Those three answers keep the next cold morning from starting at zero. If they recommend replacement, ask what failed and why a repair will not hold. Keep the old part’s name in your notes. The call is for the pattern you already wrote down, not for a tour of equipment you do not have.